1. FIELD OF THE INVENTION
The present invention relates to an oil seal, and more particularly to an oil seal which is suitable for sealing the gaps between a crankcase and a crankshaft of a crankcase pre-compression type two-cycle internal combustion engine.
2. DESCRIPTION OF THE PRIOR ART
An example of a conventional oil seal provided for an internal combustion engine of this type is so structured that it is integrally incorporated into a bearing which supports a crankshaft in such a manner as to allow the crankshaft's rotation relative to a crankcase. Since an oil seal with such a structure is manufactured on the basis of a special specification, its manufacture involves high production costs and a great deal of time. In addition, the bearing suffers from incomplete sealing of the radially inner and outer peripheral surfaces, and also from a relatively large weight as a bearing.
Another conventional oil seal of this kind is known. This oil seal has at a cup-shaped supporting member having at the radially outer edge a peripheral wall which is integral therewith and extends in the axial direction, and an annular elastic sealing member supported on the radially inner end of the supporting member, the tip of the axially extending peripheral wall of the peripheral supporting member being supported in such a manner as to face the axially inner direction of the device. Such an oil seal forms, on the crankcase side, a relatively large dead space communicating with the interior of the crankcase, which in turn causes an increase in the volume of the interior of the crankcase. Consequently, in the case of a crankcase pre-compression type two-cycle internal combustion engine, the primary compression ratio of the mixture within the crankcase drops, thus failing to provide the desired engine efficiency.